Very little is known about the molecular mechanisms that underlie the pathophysiology of leiomyoma uteri. In contrast to carcinomas which are generally singular and for which the stages of tumorigenesis and associated molecular alterations are well described, leiomyoma uteri are usually multiple and events that determine their long-term sequelae have not been elucidated. Furthermore, the natural history of individual leiomyomas within a single woman may be quite different. This indicates that even though tumors within a given woman are exposed to the same hormonal mileu, inter-tumor heterogeneity in hormone responsiveness must exist. Such inter-tumor heterogeneity may be the result of differences in molecular etiology, which could also determine responsiveness to hormonal therapy and the impact of environmental estrogen exposure. Loss of the tuberous sclerosis complex 2 (TSC-2) tumor suppressor gene results in the development of leiomyoma uteri in the Eker rat model for this disease. However, defects in TSC-2 or the pathways in which this tumor suppressor gene participate have not been investigated in human leiomyomas. Recently, the TSC- 2 gene product tuberin has been shown to play an important role in PI3K signaling, which can impact both estrogen receptor signaling and cell cycle control via p27. Our preliminary data from the Eker rat model and translational studies utilizing primary human tumors suggest that TSC-2 and p27 play important, and possibly inter-related, roles in the molecular etiology of leiomyoma uteri. Recognition of the involvement of TSC-2, PI3K signaling and p27 in the etiology of leiomyoma uteri now provides a unique avenue for understanding the molecular basis of aberrant cell cycle regulation and hormonal responsiveness in this disease. The goal of this proposal is to identify the mechanisms responsible for differential cell cycle regulation in leiomyoma uteri that may underlie inter-tumor heterogeneity and responsiveness. To accomplish this goal, the Specific Aims of this proposal are: 1) Determine the mechanism responsible for diminished p27 function in leiomyoma uteri and the role of PI3K signaling and tuberin in this process; 2) Test the hypothesis that alteration of p27 expression levels is a critical regulator of normal and neoplastic myometrial cell growth that can determine natural history of this disease; and 3) Determine if diminished p27 levels and/or aberrant P13K/AKT signaling enhances estrogen-receptor signaling in leiomyomas and potentiates the activity of SERMs and environmental estrogens in these tumors. Data generated from these experiments will help elucidate how defective cell cycle regulation and estrogen receptor signaling to contribute to the pathophysiology of leiomyoma uteri.